1. Field of the Invention
The present invention relates to an adjustable surface coil of the type suitable for obtaining nuclear magnetic resonance signals from an examination subject in a nuclear magnetic resonance examination apparatus.
2. Description of Prior Art
Nuclear magnetic resonance examination devices may use surface coils to obtain localized images of an examination subject. Surface coils can generate both fundamental and gradient magnetic fields applicable to the examination subject. The subject is irradiated via an antenna with a sequence of radio frequency pulses, resulting in the emission of nuclear magnetic resonance signals, which are acquired by a surface coil, and transmitted to an evaluation unit through a tuning circuit. Such a surface coil is disclosed, for example, in published patent application WO 84/00214.
Surface coils are used in nuclear magnetic resonance tomography, as well as nuclear magnetic resonance spectroscopy, when localized images require an improved image contrast. Such contrast can be attained by improving the signal-to-noise ratio of the emitted signals from the examination subject. For example, in spinal column diagnosis, a compromise must be made using conventional surface coils. Large (oval) surface coils will allow for a large measuring field to be examined with a high penetration depth. The signal-to-noise ratio for such a large field, however, is poor. For small (generally round) surface coils, the signal-to-noise ratio is favorable, however, only a small measuring field and a slight penetration depth can be attained.
A surface coil having numerous conductor sections arranged relative to each other, with switches connecting the conducting sections in different combinations, permits the use of a single surface coil to be used in the examination of different localized regions without physically displacing the coil. In such a coil, which is disclosed in European Application 0 280 908, the different combinations of conductor sections, when connected, may form a rectangle which is adjacent to, or overlaps, a rectangle of unconnected sections. This method of examination, however, does not result in a surface coil configuration in alignment with the geometry of the spine. As a result, an improved image contrast is still desired for localized examinations of this type.